Paraffin fuel cell

ABSTRACT

The present invention provides a fuel cell in which electricity is generated and a paraffin is converted to an olefin. Between the anode and cathode compartment of the fuel cell is a ceramic membrane of the formula BaCe 0.85-e L f Y 0.05-0.25 O (3-δ)  wherein L is a lanthanide and f is from 0 to 0.25 and δ is the oxygen deficiency in the ceramic.

FIELD OF THE INVENTION

The present invention relates to the conversion of ethane to ethylene ina fuel cell and thereby also to generate electricity and water.

BACKGROUND OF THE INVENTION

There are a number of patents which disclose fuel cells having apolymeric membrane. These include for example WO 02/38832 published May16, 2002 in the name of the University of Alberta. This type ofreference fails to disclose a ceramic suitable for use as a membrane ina fuel cell.

U.S. Pat. No. 5,139,541 issued Aug. 18, 1992 to Edlund assigned to BendResearch, Inc. discloses a composite membrane for use in separationpurification of hydrogen. The membrane comprises two non porous hydrogenpermeable foils or membranes about 30 microns thick separated by anintermetallic (foil) barrier layer which prevents metallic diffusionbetween the two foils. The patent does not teach or suggest ceramicmembranes or electrolyte.

U.S. Pat. No. 6,125,987 issued Nov. 28, 2000 to Ma, et al. assigned toWorcester Polytechnic Institute is similar except one of the metalmembranes is a porous metallic membrane. Again the patent teachesagainst ceramics.

U.S. Pat. No. 5,229,102 issued Jul. 20, 1993 to Minet, et al. assignedto Medalert, Inc. teaches a steam reforming process conducted inside aheated metal ceramic. The ceramic is alumina. The patent fails to teacha fuel cell nor does it teach converting ethane to ethylene. The patentteaches the reformatting of methane to mainly carbon monoxide andhydrogen. The reference teaches away from the present invention.

U.S. Pat. No. 6,821,501 issued Nov. 23, 2004 to Matzokos, et al.assigned to Shell Oil Company teaches a fuel cell using a ceramicsupport for the membrane. The ceramic support is typically alumina. Themembrane is typically a group VIII metal, preferably Pd and Pd alloys.The feed is a vapourizable hydrocarbon and the off gas is largelyhydrogen and CO₂ without generating an ethylene. The reference teachesaway from the subject matter of the present invention.

There are a number of papers which disclose the use of BaCeO₃ doped withabout 15% of Y (BCY 15) as a proton conducting membrane for thedehydrogenation of propane to propylene with the production ofelectricity and water. The papers include:

Yu Feng, Jingli Luo, Shouyan Wang, Juri Melnik and Karl T. Chuang,“Investigation of Y-doped BaCeO₃ as Electrolyte in Propane Fueled ProtonConducting Solid Oxide Fuel Cell”, Proceedings of the Fuel Cell andHydrogen Technologies, D. Ghosh, Edt. 44^(th) Annual Conference ofMetallurgists of CIM, MET SOC, Montreal, Quebec, pp. 461-472, 2005. (YuFeng presented this paper in the symposium of Fuel Cell and HydrogenTechnologies, 44^(th) annual Conference of Metallurgists of CIM,Calgary, August 2005); andYu Feng, Jingli Luo, and Karl T. Chuang, “Analysis and Improvement ofChemical Stability of Y-Doped BaCeO₃ as Proton-Conducting Electrolytesin C₃H₈—O₂ Fuel Cells”, which was presented at the 6th InternationalSymposium on New Materials for Electrochemical Systems, Montreal, Jul.9-12, 2006. As requested by the conference, the manuscript was submittedto the Journal of New Materials for Electrochemical Systems in May 2006.

These papers do not disclose the conversion of ethane to ethylene. As apractical matter if a fuel cell works with a paraffin it may work withhigher paraffins (e.g. works for propane it will likely work for butane)but it is much more uncertain if it will work with a lower paraffin(works with propane not sure if it would work with ethane).

The paper “Conversion of Propane to Propylene in a Proton ConductingSolid Oxide Fuel Cell” by Yu Feng, Jingli Luo, and Karl T. Chuang, to bepublished in Fuel by Elsevier, also only discloses the use of BCY15 as amembrane. These papers do not disclose the subject matter of the presentinvention.

The present invention also seeks to provide a process for convertingethane to ethylene in a fuel cell having BCY 15 or lanthanide doped BCY15 as a ceramic membrane.

SUMMARY OF THE INVENTION

The present invention provides ceramic perovskite, consistingessentially of BaCe_(0.85-e)L_(f)Y_(0.05-0.25)O_((3-δ)) wherein L is alanthanide and f is from 0 to 0.25 and δ is the oxygen deficiency in theceramic.

The present invention provides a ceramic membrane of the above ceramicand a fuel cell in which the ceramic membrane separates the anodecompartment and the cathode compartment.

The present invention further provides a process to generate anelectrical current comprising feeding to the anode compartment of a fuelcell comprising an anode compartment and a cathode compartment andhermetically sealed there between an electrolytic proton conductingceramic membrane of the formulaBaCe_(0.95-0.75)Y_(0.05-0.15)L_(f)O_((3-δ)) where L is a lanthanide andf is from 0 to 0.2 and δ is the oxygen deficiency in the ceramic at atemperature from 500° C. to 900° C. a gaseous stream comprising at least75 weight % of ethane and removing from the anode compartment a streamcomprising unreacted ethane and the resulting alkene at least 80% ofwhich is ethylene, feeding to the cathode compartment of said fuel cella gaseous stream comprising at least 20 weight % of oxygen and removingfrom the cathode compartment water and unreacted cathode compartmentfeedstream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch of a fuel cell in accordance with the presentinvention.

DETAILED DESCRIPTION

As used in this specification the phrase oxygen vacancy of the ceramicmeans that the number of oxygen ions present in the crystal latticestructure of the ceramic is less than that which would be present in awell ordered and complete lattice. In the case of an oxygen deficiency,the number of oxide ions is less than that needed to balance the totalnumber of positive charges of all metal atoms of the parent structure ifthey were all present in their normal oxidation states. This can beachieved in three ways: partial substitution of a lower oxidation stateion for a higher oxidation state ion, or partial reduction of a fractionof the high oxidation state ions to a lower oxidation state, orsubstitution for an ion of higher charge with one of lower charge, forexample M⁴⁺ replaced by a different M²⁺. There are three consequences.The formula of the ceramic deviates from the stoichiometric formula ofthe parent structure as there are less than the expected number of oxideions. There are vacant sites spaced throughout the crystal latticestructure of the ceramic at which there would normally be expected to bean oxide ion. In order to balance the charges on the ions, some of themetal ions have a lower oxidation state than would occur in thestoichiometric formulation of the parent structure.

The ceramic compositions used in the present invention are prepared frommetal oxides or, in some cases, materials from which a metal oxide canbe generated such as the corresponding carbonate. Typically metal oxidesor precursors having a purity not less than 95%, preferably not lessthan 98%, most preferably not less than 99.9% are ball milled in ahydrocarbon diluent such as one or more lower (C₆₋₁₀) alkanes(paraffins) or iso-paraffins such as the ISOPAR® series of products, orC₁₋₁₀ alcohols, for a time from 18 to 36 hours, preferably from 20 to 28hours most preferably from 22 to 26 hours. One useful diluent isiso-propanol. The resulting slurry is dried and the sintered in air at atemperature from about 1400° C. to about 1700° C., preferably from 1500°C. to about 1600° C., most preferably from 1525° C. to about 1575° C.for from about 1 to 5 hours, typically 2 to 4 hours, to produce a singlephase compound. The resulting powder is then pressed at conventionalpressures (e.g. from at least 20 MPA, typically at least 30 MPA) toproduce a green ceramic part (membrane) and sintered as described above,to produce a part having at least 90%, preferably 95%, of thetheoretical density. The starting oxides, or carbonates from which saidoxides can be derived, may be selected from the group consisting ofBaCO₃, CeO₂, and Y₂O₃. Optionally, if a porous material is desiredrather than a high density material for use a component of the electrodematerial, pore formers such as corn starch, graphite, and finely groundpolymers such as poly(methyl methacrylate) or polyethylene may beincluded in the ball milling step or the compression step. A combinationof up to about 35 weight % of one or more pore formers may be used suchas up to 16% weight % of corn starch and up to 16 weight % graphitebased on the final weight of the composition prior to further sintering.A preferred pore size in the finished ceramic part is from 1 to 5 μmpreferably from 2 to 3 μm. The ratio of the above noted oxides isselected to give the required empirical formula for the ceramic.

The ceramic in accordance with the present invention has the formulaBaCe_(0.95-75)Y_(0.05-0.15)L_(f)O_((3-δ)) where L is a lanthanide and fis from 0 to 0.2 and δ is the oxygen deficiency in the ceramic. Apreferred lanthanide is Pr. Preferably, the lanthanide dopant is Pr andf is from 0.10 to 0.2.

Referring to FIG. 1, the resulting sintered part is a membrane 11 theopposed surfaces 13, 14 of which typically are ground and will act aspart of the anode chamber 9 or cathode chamber 10 of a fuel cell 100.The membrane surfaces are first ground to remove segregated surfaceoxides arising from the sintering such as CeO₂, and PrO₂, and to reducethe thickness to the appropriate size. The thickness of membrane 11should be minimized to optimize performance of fuel cell 100, but shouldbe sufficiently thick so as to be strong enough to sustain physicalintegrity. In laboratory applications membrane 11 may have a thicknessfrom about 0.5 to 2 mm, preferably from about 0.5 to 1 mm. In industrialapplications membrane 11 could be much thinner.

An electrode 3, 4 is applied to each of opposed faces 13, 14 of ceramicmembrane 11 which will be used in fuel cell 100. Generally cathode 4includes a catalyst selected from oxygen activation catalysts and anode3 includes catalysts selected from the group consisting of hydrocarbonactivation catalysts. The electrode material used in the presentinvention typically is prepared as a paste. The electrode for both anode3 and cathode 4 may be a precious metal such as Pt or Pd, preferably Ptpaste. Platinum paste is commercially available for example from HereausInc., CL-5100. The anode catalyst may be selected from the groupconsisting of platinum, mixtures of copper and copper chromite, andmixtures of iron, platinum and chromia. To prepare 48% Fe-4% Pt-48%Cr₂O₃ catalyst, firstly nano Cr₂O₃ powder is added to a 0.5M Fe(NO₃)₂solution with electromagnetic stirring. After the solvent has beenevaporated under low heat (e.g. temperature less than 150° C.,preferably less than 120° C.), the resulting dry powder is added to asolution of tetra-ammine-platinum nitrate (5% Pt) with electromagneticstirring. This mixed solution is heated, on low heat as described aboveto evaporate solvent and produce dry powder, which is reduced in flowingH₂ at 300° C. for 30 hours to form 48% Fe-4% Pt-48% Cr₂O₃ anodecatalyst. The anode and cathode catalysts may be applied to the faces ofthe ceramic membrane by any suitable means. One method is by screenprinting to provide an electrode catalyst surface. The surface is driedat from room temperature to temperatures up to 120° C. overnight. Ifdesired a mesh may be placed over the electrode catalyst to collectcurrent.

As shown in FIG. 1, fuel cell 100 comprises an anode chamber orcompartment 9 and a cathode chamber or compartment 10 having therebetween ceramic membrane 11 coated at opposed faces 13, 14 with theappropriate anode electrode catalyst 3 and cathode electrode catalyst 4respectively. Anode chamber 9 and cathode chamber 10 are hermeticallysealed using a high temperature ceramic sealant 1, 2 about ceramicmembrane 11 described above. A number of sealants are known but ceramicsealers such as AREMCO® 503 and most preferably glass sealants such asAREMCO® 617 may be used to hermetically seal fuel cell compartments 9and 10.

Fuel cell 100 generally operates at a temperature from 500° C. to 900°C., preferably 600° C. to 800° C. Heat may be provided by anyconventional source such as electric heaters or fired heaters. To someextent this may depend on the feed and its heat value.

Cathode compartment 10 is fed with cathode feed stream 5 comprising atleast 20 weight % of oxygen. Preferably cathode compartment 10 is fedwith stream 5 comprising a higher amount of oxygen typically greaterthan 60 weight % preferably greater than 75 weight % most preferablygreater than 90 weight % oxygen most desirably greater than 95 weight %of pure oxygen. The feed to the cathode compartment may be lightlyhumidified. It may comprise from about 5 to 10% more water vapour thanin the ambient environment. The exhaust stream 6 from the cathodecompartment 10 comprises water vapor and unconsumed cathode feed gas.

The feed and exhaust ports may be any of a number of well known designs.There could be separate spaced apart ports for the feed and exhaust orthe ports could be provided by concentric ports with oxygen feed 5directed towards the central part of the cathode electrode catalyst 4and exhaust stream 6 being drawn off from the periphery of anode 4.

The anode feed stream 7 to anode compartment 9 may comprise at least 75,preferably 80 weight % of ethane. Most preferably the ethylene feed isquite pure, preferably over 90 weight %, most preferably over 95 weight%. One of the advantages of the process of the present invention isselectivity (e.g. ethane feed gives ethylene and ethane). When anodefeed stream 7 is a relatively pure ethane stream, anode exhaust stream 8also contains essentially only ethylene and no significant amounts ofother alkenes. This reduces the energy costs to separate close alkenes(e.g. the compressor costs and cost of cryogenic separation to separatemethane from ethylene from propylene).

Anode feed stream 7 is normally dry. The atmosphere in cathodecompartment 10 is partially humidified by product water. It was foundthat the performance of the fuel cell was improved by the presence oflight humidification.

EXAMPLES

The present invention will now be illustrated by the followingnonlimiting examples.

Example 1 Components and Preparation

Compositions of BaCe_(0.85)Y_(0.15)O_((3-δ)) (BCY 15) were prepared asfollows.

Polycrystalline powders of BCZ 15 were synthesized from high purityBaCO₃ and nanopowders of CeO₂, and Y₂O₃ in amounts to give the requiredformula that were mixed, ball-milled and the resulting raw mixes werecalcined at 1350° C. for 10 hours in air. The resulting materials wereagain ball-milled, pressed into disks (20 mm diameter) and sintered at1600° C. for 12 hours in air. The sintered samples normally haddensities in the range 90-96% of theoretical values, as determined fromtheir weights and volumes. Minor loss of BaO during sintering resultedin the formation of CeO₂ and PrO₂ on the surfaces. Consequently, surfacelayers which contained the decomposed material were removed by polishingboth sides of the membrane. Screen printed platinum electrodes wereapplied to each face of the membrane.

To prepare 48% Fe-4% Pt-48% Cr₂O₃ catalyst, firstly nano Cr₂O₃ powderwas added to a 0.5M Fe(NO₃)₂ solution with electromagnetic stirring withmild heating. After the solvent had evaporated, the resulting dry powderwas added to a solution of tetra-ammine-platinum (II) nitrate (5% Pt)with electromagnetic stirring. This mixed solution was heated toevaporate solvent and produce dry powder, which then was reduced inflowing H₂ at 300° C. for 30 hours to form 48% Fe-4% Pt-48% Cr₂O₃.

Example 2 Stability of BCY 15

Thermogravimetric analysis (TGA) showed that BCY(BaCe_(0.85)Y_(0.15)O_((3-α)) reacts with CO₂ to form carbonate attemperatures over 500° C. The carbonate components of mixtures so formedfrom BCY lose CO₂ at temperatures over 1050° C.

Example 3

A simple fuel cell 100 was prepared by sealing a tube 16,17 onto each ofthe opposed faces 13,14 of the prepared ceramic membrane 11 with Ptcatalysts/electrodes 3, 4 on the respective surfaces 13,14. Anapproximately concentric inner tube 18,19 was then inserted into each offirst tubes 16,17 to act as a feed tube. Outer tubes 16,17 acted as thecorresponding exhaust tubes or ports. Current collectors 21, 22 wereattached to each catalyst/electrode 3, 4 and were used to measurecurrent and current density. The entire cell 100 was placed in an oven(not shown) heated to various temperatures and ethane was the anode feedstream 7 fed to anode 3 in anode compartment 9 and 20% oxygen was thecathode feed stream 5 fed to cathode compartment 10.

The flow rate of C₂H₆ and O₂ was 100 mL/min.

For a BCY15 electrolyte fuel cell operated at 700° C., the ethaneconversion and ethylene selectivity were 33.7% and 96.3%, respectively.

Example 4

The C₂H₆—O₂ fuel cell as above, except that the anode electrode/catalystwas iron and platinum mixed with nano-Cr₂O₃ for electrode catalyst asprepared above, showed a steady OCV of 1.08 V at both 650° C. and 700°C. At 650° C., C₂H₆—O₂ fuel cell using the anode catalyst delivered amaximum power density of only 47 mW/cm² and a corresponding currentdensity of 78 mA/cm². When the fuel cell was operated at 700° C., themaximum power density was improved to 243 mW/cm² and the correspondingcurrent density also was enhanced to 540 mA/cm². This cell performanceimprovement was attributed to the reduction of cell impedance from 26.8Ohm at 650° C. to 10.8 Ohm at 700° C.

The foregoing examples demonstrate the feasibility of the presentinvention.

1. A ceramic perovskite, consisting essentially of:BaCe_(0.95-0.75)Pr_(f)Y_(0.05-0.25)O_((3-δ)) wherein f is from 0.1 to0.20 and δ is the oxygen deficiency in the ceramic.
 2. A fuel cellcomprising an anode compartment and a cathode compartment andhermetically sealed there between an electrolytic proton conductingceramic membrane having a thickness from 0.5 to 2 mm as defined inclaim
 1. 3. A ceramic membrane having a thickness from 0.5 to 2 mm ofthe formula of claim 1.